The invention relates to an identification transponder for providing an identification code to a base station through variation of a radio frequency signal originating from the base station in a rhythm corresponding to the identification code, comprising identification generation means for generating the identification code; rectifying means constructed so as to achieve a coupling without direct electrical contact with the radio frequency signal for providing a supply voltage to the identification generation means, which rectifying means comprise at least one circuit acting as a rectifier; and a load transistor with a main current path and with a control electrode which is coupled to a signal output terminal of the identification generation means for providing a varying load to the rectifying means as a result of which the radio frequency signal varies in said rhythm.
Such an identification transponder is known from U.S. Pat. No. 6,133,835, issued Oct. 17, 2000 to Dagobert M. De Leeuw, Cornelis M. Hart, and Marco Matters, and incorporated by reference herein. The known identification transponder is shown in FIG. 1 together wit a base station or scanner. The base station BS transmits an RF signal that is generated by an oscillator OSC, inductor L, and capacitor C2. The RF signal is supplied to a rectifier in the transponder by means of a coupling (P1;P3, P2;P4), a capacitive coupling in this case, which rectifier is built up with diodes D1 and D2 and a smoothing capacitor C1. As a result, a supply voltage is available between the output terminals VP and MN of the rectifier. The known identification transponder IDT further comprises an identification code generator IDOG which comprises a so-called read-only memory ROM and a modulator MOD, which are both supplied from the supply voltage between the terminals VP and VN. The known identification transponder IDT further comprises a load transistor TL which is coupled with a main current path between the terminals VP and VN. During operation, the modulator MOD sends a serial bit pattern to a control electrode of the load transistor TL. The serial bit pattern is determined by the data stored in the memory ROM. As a result, the load between the terminals VP and VN is varied in the rhythm of the serial bit pattern. This causes the input current of tbe rectifier to be varied in said rhythm. This rhythm can be detected by the base station BS by means of the coupling (P1;P3, P2;P4), and decoded via a demodulator DEMOD. The ROM may be, for example, a bar code on an object which contains information about the object, for example the price of the object. This information is transmitted to the base station BS via a temporary coupling to the base station BS. The transponder IDT may be costructed, for example, as part of an integrated circuit manufactured from organic materials, for example polymers. This has the advantage that an identification transponder can be realized thereby with a small thickness and a high mechanical flexibility.
Generally, but in particular in the realization of electronic circuits with organic materials, it is advantageous to minimize the number of electronic components required. The present invention meets this requirement in that the identification transponder mentioned in the opening paragraph is characterized in that one of the circuits acting as said rectifier comprises the main current path of the load transistor.
Since the load transistor performs a dual function, i.e. the function of providing a variable load and the function of a rectifying element, one electronic component less is necessary. Indeed, the rectifier requires one diode less, and no additional components are added elsewhere in the electronic circuit.
Further advantageous embodiments of the invention are specified in the claims 2 to 6.